Our invention is adapted to be used in an automatic transmission for an automotive vehicle having an internal combustion engine, multiple-ratio gearing and a hydrokinetic torque converter located between the engine crankshaft and the torque input elements of the gearing. It is adaptable also for use with a neutral clutch for a continuously variable transmission or for certain semi-automatic transmissions. The output elements of the gearing are adapted to be drivably connected to vehicle traction wheels. An example of a transmission of this kind is disclosed in U.S. Pat. No. 4,526,065 and in U.S. Pat. No. 4,476,745. Each of these patents is assigned to the assignee of the present invention.
The transmission disclosed in the '745 and the '065 patents includes an internal combustion engine having a crankshaft connected to the impeller of a hydrokinetic torque converter. Multiple-ratio gearing provides plural torque flow paths between the turbine of the hydrokinetic torque converter and the vehicle traction wheels. Fluid pressure-operated clutches and brakes establish and disestablish torque flow paths between the turbine and driven elements of the gearing. Fluid pressure-operated servos actuate the clutches and brakes.
The servos for controlling the clutches and brakes include a forward-drive clutch servo that is actuated during operation in each underdrive ratio as well as the direct-drive ratio. The torque of the engine is multiplied by the converter and distributed to the torque input elements of the gearing through the forward-drive clutch. This is true even in those instances when the vehicle is idling. Thus, a torque flow path is established when the vehicle is not moving but when the engine is idling and the transmission is in the forward drive position.
The control systems of the '745 patent and the '065 patent include driver-operated range selector valves which have park or neutral positions as well as a forward-drive position, each position being established by a driver-operated manual valve detent. The torque flow path can be interrupted by adjusting the driver-operated manual valve to the neutral position, thereby disengaging the forward-drive clutch.
If the vehicle engine should be idling when the vehicle is stopped and if the manual valve is moved to the drive position, the engine will be loaded because the impeller of the converter will continue to be connected to the crankshaft of the engine as the turbine is stalled. Thus there is a speed ratio developed across the converter, although the torque delivered through the converter is at a minimum value because of the idle condition of the engine.
In order to avoid torque disturbances and roughness in the idle state, it is necessary for the engine to be calibrated so that the engine control strategy will provide sufficient fuel to maintain a smooth idle while the vehicle is stalled and the manual valve is in the drive range position. The noise, vibration and harshness (NVH) of the powertrain may be severe because of the direct torque flow path between the turbine and the driven elements of the transmission in contrast to the NVH characteristics for the full neutral condition when the manual valve is in the neutral position.
The requirement to increase the fuel supply by advancing the throttle during idle condition reduces the effective fuel economy of the driveline. Further, undesirable exhaust emission levels are possible when the driveline is loaded in this fashion with the engine in its idle condition.